A. Human IGF (Insulin-like Growth Factor)
Human IGF has been the subject of a fair amount of intensive study by past workers. A body of literature has been developed related to various aspects of this protein or series of proteins (see references A through L).
Insulin-like growth factors I and II have been isolated from human serum (A). The designation "insulin-like growth factor" or IGF was chosen to express the insulin-like effects and the insulin-like structure of these polypeptides which act as mitogens on a number of cells. The complete amino acid sequences of IGF-I and IGF-II have been determined (D,E). They are both single-chain polypeptides with three disulphide bridges and a sequence identity of 49 and 47 percent respectively, to human insulin A and B chains. The connecting peptide or C region is considerably shorter than the one of proinsulin and does not show any significant homology to it. (For a summary of earlier studies on the biological efforts of IGF, see Reference F).
IGF-I and IGF-II are growth promoting polypeptides occurring in human serum and human cerebral spinal fluid. Their structure is homologous to proinsulin. IGF-I seems to be produced by the liver along with a specific IGF-binding protein both of which are under control of growth hormone. Thus, human IGF is considered to be an active growth promoting molecule that mediates the effect of human growth hormone.
It was perceived that the application of recombinant DNA and associated technologies would be a most effective way of providing the requisite large quantities of high quality human IGF for applied use to human beings as a growth factor. The goal was to produce human IGF either as biologically active fusion protein, or more importantly, as a mature protein, as products of recombinant DNA technology from a host organism. Such materials would exhibit bioactivity admitting of their use clinically in the treatment of various growth affected conditions.
B. Recombinant DNA Technology
Recombinant DNA technology has reached the age of some sophistication. Molecular biologists are able to recombine various DNA sequences with some facility, creating new DNA entities capable of producing copious amounts of exogenous protein product in transformed microbes and cell cultures. The general means and methods are in hand for the in vitro ligation of various blunt ended or "sticky" ended fragments of DNA, producing potent expression vehicles useful in transforming particular organisms, thus directing their efficient synthesis of desired exogenous product. However, on an individual product basis, the pathway remains somewhat tortuous and the science has not advanced to a stage where regular predictions of success can be made. Indeed, those who portend successful results without the underlying experimental basis, do so with considerable risk of inoperability.
DNA recombination of the essential elements, i.e., an origin of replication, one or more phenotypic selection characteristics, an expression promoter, heterologous gene insert and remainder vector, generally is performed outside the host cell. The resulting recombinant replicable expression vehicle, or plasmid, is introduced into cells by transformation and large quantities of the recombinant vehicle are obtained by growing the transformant. Where the gene is properly inserted with reference to portions which govern the transcription and translation of the encoded DNA message, the resulting expression vehicle is useful to actually produce the polypeptide sequence for which the inserted gene codes, a process referred to as expression. The resulting product may be obtained by lysing, if necessary, the host cell, in microbial systems, and recovering the product by appropriate purification from other proteins.
In practice, the use of recombinant DNA technology can express entirely heterologous polypeptides-so-called direct expression-or alternatively may express a heterologous polypeptide fused to a portion of the amino acid sequence of a homologous polypeptide. In the latter cases, the intended bioactive product is sometimes rendered bioinactive within the fused, homologous/heterologous polypeptide until it is cleaved in an extracellular environment. See references (M) and (N).
Similarly, the art of cell or tissue cultures for studying genetics and cell physiology is well established. Means and methods are in hand for maintaining permanent cell lines, prepared by successive serial transfers from isolated normal cells. For use in research, such cell lines are maintained on a solid support in liquid medium, or by growth in suspension containing support nutriments. Scale-up for large preparations seems to pose only mechanical problems. For further background, attention is directed to references (O) and (P).
Likewise, protein biochemistry is a useful, indeed necessary, adjunct in biotechnology. Cells producing the desired protein also produce hundreds of other proteins, endogenous products of the cell's metabolism. These contaminating proteins, as well as other compounds, if not removed from the desired protein, could prove toxic if administered to an animal or human in the course of therapeutic treatment with desired protein. Hence, the techniques of protein biochemistry come to bear, allowing the design of separation procedures suitable for the particular system under consideration and providing a homogeneous product safe for intended use. Protein biochemistry also proves the identity of the desired product, characterizing it and ensuring that the cells have produced it faithfully with no alterations or mutations. This branch of science is also involved in the design of bioassays, stability studies and other procedures necessary to apply before successful clinical studies and marketing can take place.